This invention relates generally a to method and apparatus for controlling an internal combustion engine in accordance with various information including atmospheric pressure.
A speed density control system (referred to as S-D system) is known for controlling an internal combustion engine, such as an engine of a motor vehicle. This system calculates a necessary amount of fuel using two parameters. These parameters include one parameter indicative of the air pressure in the intake manifold, and another parameter indicative of the rotational speed of the engine crankshaft.
When an internal combustion engine is controlled with such a conventional S-D system without effecting a feedback control, the air/fuel ratio of the air/fuel mixture supplied to the engine is apt to be deviated toward the lean side when atmospheric pressure (Patm) decreases when a motor vehicle is driven at a high altitude. This is because it cannot be unequivocably determined that exhaust manifold pressure (Pr) decreases and volumetric efficiency (.eta.v) increases when the amount of fuel is determined on the basis of only the intake manifold pressure (Pm) and engine speed (N). This fact is indicated by the following formula (1), and as a result of such deviation in air/fuel ratio, the drivability and engine starting condition become poor. EQU .eta.v.varies.(Pm/Patm)X[1+(1-Pr/Pm)/(K.sup.(.epsilon.-1) ](1)
wherein
K is a constant; and PA1 .epsilon. is a compression ratio.
In order to compensate for such undesirable deviation of the air/fuel ratio toward the lean side, an atmospheric pressure sensor or an exhaust gas pressure sensor has been employed independent of the intake manifold pressure sensor. However, such as engine control system suffers from an increase in cost inasmuch as it requires two separate pressure sensors, while high reliability is not ensured because of variations in accuracy between two separate sensors.